Authigenic K-feldspar in British Permo-Triassic sandstones

Authigenic K-feldspar, occurring as overgrowths around detrital orthoclase and microcline, is a common minor diagenetic constituent in British Permo-Triassic sandstones. Overgrowths began as crystallographically oriented rhombs (∼10 μ m) with an adularia habit. By merging, stacking and overlapping, these eventually produced complete overgrowths with crystalline perfection. The overgrowths possess extraordinary chemical purity, being stoichiometric KAlSi 3 O g . This together with − 2 V values of 20–25°, suggests that the authigenic phase is potassian intermediate sanidine. The source of the K 2 O, A1 2 O 3 and SiO 2 is ascribed to the in situ dissolution of detrital K-feldspar, plagioclase, biotite and other silicates. The intrastratal solutions thus produced also provided authigenic quartz, illite, haematite and calcite.